If your car spends most of the week sitting in a driveway or parking garage, why not defray some of the cost of owning it by renting it out? That's the thinking behind RelayRides, a company that connects renters with the owners of idle cars.

RelayRides' connection with OnStar will make car sharing even easier for renters using OnStar-equipped GM vehicles. The satellite service, originally intended to help stranded motorists get locked-in keys out of the car, call a tow truck, and other aids to disaster mitigation, allows renters to open a shared car with a phone call or text message.

RelayRides was a year-old startup when GM's venture capital arm scooped it up in late 2011. Google's investment branch also owns a chunk of the car share company.

GM said in a statement that RelayRides customers can make "hundreds of dollars per month," and the service extends also to vehicles that aren't equipped with OnStar. In those cases, vehicle owners and renters opt for an old fashioned, face-to-face key swap.

This could be great. Traveling to a city where you probably need a car (and where going to the airport to collect one isn't an attractive option) but can't have one can be such a pain in the ass. Now, you can just borrow some random schmo's car for a little bit.